The present invention is directed to a system and method for recovering energy in a hydraulic circuit. More particularly, the invention relates to a system and method for recovering energy in a hydraulic circuit.
In a machine, such as an excavator or a loader, a hydraulic circuit may include a variable displacement pump in fluid communication with a hydraulic actuator to handle a variable load. The pump provides pressurized fluid to the hydraulic actuator, such as a hydraulic cylinder, to lift the load. The actuator may be connected to an implement, such as a bucket.
When the load is lowered, the pressurized fluid in the hydraulic actuator is often discharged from the actuator to a reservoir. There is energy in discharging the pressurized fluid from the hydraulic actuator when lowering the load. However, many machines have no means of recovering the energy when the hydraulic actuator is retracted. Typically, these machines throttle the fluid through a valve to control a lowering or retracting speed of the actuator. This results in a loss or waste of energy and undesired heating of the hydraulic fluid.
The above situation can occur, for example, when a hydraulic cylinder is operated under an overrunning load. After a hydraulic cylinder has been extended to lift the load, the cylinder may retract by itself due to its own weight. This is often referred as an overrunning load condition. Overrunning load conditions can be readily observed during machine operation.
Some attempts have been made to recover this otherwise wasted energy in the hydraulic circuit. For example, WO 00/00748 discloses a system that recovers energy by providing an additional pump/motor with an over-center capability in the hydraulic circuit. The pump/motor transfers fluid between a lifting circuit and an accumulator for storing energy. However, such an accumulator increases the size of the machine. Also, when the lifting cylinder is dropped rapidly, a large quantity of fluid is discharged rapidly from the cylinder. To accommodate the fluid, the pump/motor needs to be large. The disclosed system also requires an additional charge pump and a valve to fluidly couple the pump/motor to the lifting cylinder. Such a charge pump is not energy efficient, and the additional components increase the cost of the machine system. The system has another shortcoming that when the lifting cylinder is being retracted and the accumulator is at a higher pressure than the fluid discharged from the lift cylinder, additional energy from the engine is required to store the energy coming from the lift cylinder.
Thus, it is desirable to provide an energy recovering system that is energy efficient and cost effective. The present invention is directed to solving one or more of the above-mentioned shortcomings.
In one aspect, a method is provided for recovering energy in a hydraulic circuit. The hydraulic circuit includes a pump having a swashplate and being in fluid communication with a hydraulic actuator via a valve. The method includes sensing an overrunning load condition in the hydraulic circuit, actuating the valve to provide fluid from the hydraulic actuator to the pump under the overrunning load condition, and producing a torque output from the fluid provided to the pump.
In another aspect, a system is provided for recovering energy in a hydraulic circuit. The system includes a pump having a swashplate tiltable to direct flow between a valve and a reservoir. A hydraulic actuator is provided in fluid communication with the pump via the valve and a conduit. The valve is configured to provide fluid from the hydraulic actuator to the pump under an overrunning load condition. A sensor assembly is provided in communication with the hydraulic circuit, and a control unit is electrically coupled to the valve and the sensor assembly.
In another aspect, a method is provided for recovering energy in a hydraulic circuit including a pump and a motor in fluid communication with a hydraulic actuator via a valve. The method includes sensing an overrunning load condition in the hydraulic circuit, actuating the valve to provide fluid from the hydraulic actuator to the motor under the overrunning load condition, and producing a torque output from the fluid provided to the motor.
In another aspect, a system is provided for recovering energy in a hydraulic circuit. The system including a pump and a hydraulic actuator in fluid communication with the pump via a valve and a conduit. A motor is provided in fluid communication with the hydraulic actuator via the valve. The valve is configured to provide fluid from the hydraulic actuator to the motor under an overrunning load condition. A sensor assembly is provided in communication with the hydraulic circuit, and a control unit is electrically coupled to the valve and the sensor assembly.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.